Guys I was just wondering... I'm about to start working up some 30-06 loads w/ some 4831 and some 180 bergers... and I'm seeing a good bit of case length differences. ALMOST all of my once fired 7mm RUMs are 2.850, with a few @ 2.845s. These 30-06 cases are ranging from 2.490-2.509. All sized with the same dies, now not all were fired from the same gun though.
I never even purchased a case trimmer, b/c of the consistency of the once fired 7mmRUM brass. Now to me just looking @ the case trimmers it looks like its going to be hard to get every single case the same lenght without repeatedly checking, and IF you do slip up and cut one a little too much it looks like your gunna start BACK over... IF you have racked up about 50-60 brass, I guess you can set that case out... BUT I like to work in even numbers! lol
I'm gunna order myself a case trimmer this week anyway.
IF loading once fired cases I always size them first, then if more than a few are on the upper end of the length, I will run them all through my trimmer set to 2.484". Some have a goodly amount cut some do not even touch, but overall they will all still shoot well within the accuracy levels of my rifles, and me on most occasions.
On new cases, I run them through the sizing die just enough to allow an 1/8" or so of the neck to get sized and straighten any imperfections out. Most I have found are generally under the minimum length when measured so I will load them up as usual, and check them all after firing as noted by MagnumManiac. Even then after sizing some will remain at or under the minimum length, depending on the load.
Bottom line with trimming is to keep the neck from being to long and getting crimped when your bolt closes on the chamber. The other is consistency. In some rifles you might see a big difference with them all being withing .002 - .005" of each other, in some depending on the load, you might not even notice a .010" spread with regards to groups, as long as you not squeezing the neck into the bullet in the chamber.
One other thing I did want to point out, and this just comes from past experience with an auto loader, you might want to look at another powder in the mid range burn rate instead of 4831. Check powders that work well in the M1 Garand, and you will be way ahead of the game. What you will find using the slower powders is the bolt will get slammed back MUCH harder with the pressure from the slower powders, than when a faster powder is used. Something along the lines of 4895 or similar might be a better choice. The one time I loaded for an auto was for a Remington and it almost was bad from the get go using 4831. Do a little checking around I know some manuals list loads specifically for the Garand and these would be a good starting place. Here are just a couple, Speer No. 13, Hornady 5th Edition.
One last thing on an auto loader, is you MAY have to use a small base die to get the fired cases to chamber reliably, especially when using brass fired in other rifles. Sometimes when fired they have a belly on one side from laying in the chamber. While this is no problem at the time, it may cause the reload to jam when loading from the magazine as the belly is now out of round with the chamber. Just a thought and something you might run into. If you notice jams I would look into picking up a SB sizing die.
I wish you well with your loads, and these were just a couple of things I have had issue with through the years. Just throw out a heads up before you load up a hundred rounds only to find they aren't going to work in your rifle.